Method of manufacturing a membrane sensor

ABSTRACT

This invention relates to a differential pressure sensor, one embodiment of which is a microphone. The differential pressure sensor comprises: a flexible membrane ( 7 ) made of conductive material, which membrane forms a first electrode of the differential pressure sensor, and a perforated plate ( 6 ) made of conductive material, which plate is essentially more rigid than said membrane ( 7 ), which plate is arranged at a distance from the membrane, and which plate forms a second electrode of the differential pressure sensor. In order to provide a differential pressure sensor with as good properties as possible, the differential pressure sensor comprises a substrate ( 4 ), a cavity ( 10 ) extending through the substrate, the walls of which cavity are formed of said substrate ( 4 ). The membrane ( 7 ) is closely connected to the walls of the cavity ( 10 ), whereby the membrane forms a dense wall in said cavity, and said perforated plate ( 6, 6′ ) has been attached to the substrate with an insulating layer ( 5 ).

This is the U.S. National Stage of International Application No.PCT/FI01/00278, which was filed in the English language on Mar. 20,2001, and which designated the U.S.

This invention relates to manufacture of a capacitive membrane sensorwith micro-mechanical manufacturing methods. A membrane sensor refers inthis application generally to a sensor by means of which signalsprocessed with electrical circuits can be formed, the signals beingresponsive to the position and/or movements of the membrane. Examples ofmembrane sensors include a differential pressure sensor and amicrophone, which is a special case of a differential pressure sensor,functioning in the acoustic zone. In the following, the invention willbe described by way of example, referring primarily to differentialpressure sensors. It is to be noted, however, that the invention canalso be utilized in other connections.

Known from the prior art, a differential pressure sensor shown in FIG. 1utilizes a silicon substrate as a perforated back plate 1 of thedifferential pressure sensor, the back plate forming a second electrodeof the differential pressure sensor. FIG. 1 shows a cross-section of adifferential pressure sensor. The perforated back plate 1 is formed of arelatively thick layer of monocrystalline substrate silicon, which hasbeen perforated by diffusion or etching in connection with themanufacture. In connection with the manufacture, an insulating layer 3and a flexible layer 2 have been grown upon the perforated back plate 1.

The weakness of this known differential pressure sensor is that itsmanufacture requires a plurality of stages and masking layers. Thegrowing of the insulating layer, among other things, requires a separatetime-consuming process stage. Further, it is difficult to make theperforation of the back plate 1 optimally dense, because it is difficultto achieve an accurate pattern required by a dense perforation throughthe thick silicon substrate (from the lower surface in FIG. 1). Asufficiently dense perforation can be achieved for the back plate 1 byetching as a first stage pits in the silicon substrate (on the uppersurface in FIG. 1), and by filling them at the growing stage of theinsulating layer 3, but this method imposes special requirements on theetching profile of the perforations of the back plate 3 and for the stepmasking of the growing process of the insulating layer 3. Further, onlysmall pits can be filled with this method.

The perforation of the back plate is of very great significance for theproperties of the differential pressure sensor, because the perforationdetermines, in practice, how easily air can flow into the space belowthe membrane 2 of FIG. 1. If the air flow is poor, the properties of thedifferential pressure sensor suffer from this.

A microphone made of two or more silicon wafers by bonding, i.e.connecting, is known from the prior art. However, the manufacture ofsuch a known microphone is very difficult.

An object of this invention is to solve the above-described problems andto provide a method which enables manufacture of a membrane sensor withimproved properties in such a manner that the manufacture is easier thanin the known solutions. These objects are achieved with the methodaccording to claim 1 of manufacturing a membrane sensor, for example adifferential pressure sensor or a microphone. It is to be noted that themethod according to the invention does not require the steps of themethod to be performed in the order presented in claim 1, but the methodsteps can be performed in a different order, as becomes apparent fromthe examples described in connection with the appended figures.

In a method according to the invention, the membrane sensor is made of alayer-structured preform having two layers, there being a layer ofinsulating material between them. The first and second layer can be suchthat they conduct electricity. One suitable preform is an SOI(Silicon-On-Insulator) wafer, which is commercially available. Making amembrane sensor of such a preform can be implemented simply by removingmaterial from the preform, whereby only the formation of a flexiblemembrane requires growing of a new material layer. The manufacture of amembrane sensor according to the invention is thus easier than in knownsolutions.

An object of the invention is further a differential pressure sensoraccording to claim 8 and a microphone according to claim 13. Thestructure of the differential pressure sensor and microphone accordingto the invention allows the perforated plate to be perforated moreclosely than before, which improves the flow of air through theperforations, whereby the result is a differential pressure sensor andcorrespondingly a microphone having improved properties. Further, thestructure of the differential pressure sensor and microphone accordingto the invention enables utilization of a novel manufacturing method,whereby the manufacturing costs of the differential pressure sensor andcorrespondingly of the microphone are reduced.

In a preferred embodiment of the differential pressure sensor andcorrespondingly the microphone according to the invention, a bridge-typestructure is utilized. Thus, the area of the perforated platefunctioning as the second electrode is substantially smaller than thecross-section of the cavity extending through the substrate. Theperforated area of the perforated plate is arranged above the middlepart of the cavity by means of arms in such a way that the perforatedarea is, in practice, situated above the middle part of the moving areaof the flexible membrane. This bridge-type structure provides theadvantage that the electrode area of the perforated plate is centredupon the most flexible middle part of membrane. Thus, the parasitic edgecapacitance can be minimized, whereby improved separating capacity andsensitivity are achieved with the structure.

In a preferred embodiment of the differential pressure sensor andmicrophone according to the invention, a second insulating layer and asecond membrane are formed upon the perforated plate. Thus, the middlepart of the perforated plate is provided with a moving area thatconnects the membranes to each other via the insulating layers in such away that the second membrane also moves with the membrane in the cavity.The second membrane forms an additional electrode in the structure,owing to which electrode, improved capacitance modulation and improvedperformance and sensitivity are achieved with the structure.

Preferred embodiments of the method, differential pressure sensor andmicrophone according to the invention become apparent from the attacheddependent claims 2 to 7, 9 to 12 and 14 to 18.

In the following, the invention will be described in greater detail, byway of example, with reference to the attached figures, of which

FIG. 1 shows the structure of a differential pressure sensor accordingto the prior art;

FIGS. 2 a to 2 c show a first preferred embodiment of a differentialpressure sensor according to the invention;

FIG. 3 shows a second preferred embodiment of a differential pressuresensor according to the invention;

FIGS. 4 a to 4 f show the first preferred embodiment of the invention;

FIGS. 5 a to 5 d show the second preferred embodiment of the invention;

FIG. 6 shows reduction of the acoustic resistance of a microphoneaccording to the invention;

FIGS. 7 a to 7 i show a third preferred embodiment of the method anddifferential pressure sensor according to the invention;

FIG. 8 shows a fourth preferred embodiment of the differential pressuresensor according to the invention; and

FIGS. 9 a to 9 f show a fourth preferred embodiment of the method anddifferential pressure sensor according to the invention.

FIGS. 2 a to 2 c show a first preferred embodiment of the differentialpressure sensor according to the invention. It is to be noted that amicrophone is one embodiment of the differential pressure sensor, andtherefore the description relating to the figures also concerns amicrophone although the invention will later be described by referringchiefly to a differential pressure sensor.

A capacitive differential pressure sensor according to the invention isshown in FIG. 2 a as a side-section, in FIG. 2 b as a top view and inFIG. 2 c seen from below. The differential pressure sensor of FIGS. 2 ato 2 c is made of an SOI wafer having an insulating layer 5 (1 to 2 μm)upon a silicon substrate 4 and a relatively thick silicon layer upon theinsulating layer, which silicon layer forms a perforated plate 6 (5 to50 μm). Up to hundreds of differential pressure sensors can be made ofone SOI wafer or SOI preform, the size of the sensors being such thatthey can be enclosed in the enclosure of an integrated circuit whendesired. Thus, the invention enables production of such an integratedcircuit that includes a differential pressure sensor or a microphone.

The differential pressure sensor comprises a flexible membrane 7, whichis manufactured of polycrystalline silicon. In order to connect thedifferential pressure sensor to an electrical circuit, metallizedcontact pads 8 have been formed for the perforated plate 6 and thesilicon substrate 4. Through the silicon substrate, a pressure-balancingcapillary 9 has been formed, which enables the pressure-balancing inmicrophone use.

The first electrode of the differential pressure sensor of FIGS. 2 a to2 c is formed of a flexible membrane 7 arranged in a cavity 10 extendingthrough the silicon substrate 4 in such a way that the membrane isjoined to the walls of the cavity. Thus, the membrane forms in thecavity 10 a partition wall moving (bending) in accordance with pressurevariations. In the embodiment of FIGS. 2 a to 2 c, there is also anelectrical contact between the membrane 7 and silicon substrate 4. Thus,the differential pressure sensor has only two parts separated from eachother, whereby not so many contacting stages are required.

The second electrode of the differential pressure sensor in FIGS. 2 a to2 c is formed by the perforated plate 6. The plate is significantly morerigid than the membrane 7. Thus, the plate 6 does not move in connectionwith pressure variations of the plate 6 but stays still and allows air(or other substance) to flow through its perforations. The variations ofthe air pressure thus result in a change in the distance between themembrane 7 and the perforated plate 6, whereby an electrical signalproportional to the change can be generated from this distance change ina manner known per se when the differential pressure sensor is connectedto an electrical circuit via the contact pads 8.

FIG. 3 shows a second embodiment of a differential pressure sensoraccording to the invention. The differential pressure sensor of FIG. 3is a bridge-type differential pressure sensor. This differentialpressure sensor corresponds to the embodiment of FIGS. 2 a to 2 c in allother aspects except for the perforations of the perforated plate 6′being in the case of FIG. 3 gathered in the area situated above themiddle part of the membrane 7. Only arms 11 in the plate 6′ extend fromthe perforated area to the outside of the imaginary extensions of thewalls of the cavity 10, as seen from FIG. 3.

The bridge-type structure of FIG. 3 provides the advantage that theelectrode area of the plate 6′ is centred upon the most flexible middlepart of the membrane 7. Thus, the parasitic edge capacitance caused bythe edge zones of the membrane can be minimized, whereby improvedseparating capacity and sensitivity are achieved.

FIGS. 4 a to 4 f illustrate a first preferred embodiment of a methodaccording to the invention.

FIG. 4 a shows a side view of an SOI wafer of a layered structure, andFIG. 4 b shows an SOI wafer after the formation of the cavity 10 as aside-section (left-hand drawing) and as seen from below (right-handdrawing).

The cavity 10 is formed by etching. At first, the rear surface of theSOI wafer is patterned litographically, for example, after which thesilicon substrate is etched until the cavity 10 extends to theinsulating layer 5 of the SOI wafer. In the etching of the siliconsubstrate, anisotropic wet-etching can be used, for instance KOH(potassium hydroxide), or TMAH, (tetramethyl ammonium hydroxide).Alternatively, deep-etching performed with plasma (ICP, inductivelycoupled plasma) can be used. The ICP etching provides the advantage thatthe pressure-balancing needed in microphone use can be implemented witha narrow groove 9, which is etched at the same stage as the cavity 10extending through the silicon substrate.

The use of ICP etching for forming a cavity extending through thesubstrate also brings about the advantage that the shape of the flexiblemembrane can be optimized. In conventional anisotropic wet-etching ofsilicon, it is necessary to be confined to rectangular membranes.However, a circular membrane, for instance, bends 20% more than a squaremembrane having the same area. Also, in a circular membrane, the borderline has been minimized relative to the area, whereby the parasitic edgecapacitance is reduced.

FIG. 4 c illustrates growing of a flexible membrane 7. The flexiblemembrane can be made of polysilicon (polycrystalline silicon) bygrowing, for example with the CVD (chemical vapour deposition) method.The flexible membrane 7 grows evenly on all vacant surfaces. Thethickness of suitable polysilicon is approximately one micrometer.Polysilicon also grows on the lower surface of the substrate 4 and onthe upper surface of the plate 6, but, according to the invention, itdoes not have to be removed.

FIG. 4 c shows that in connection with the growing of the flexiblemembrane, the polysilicon only grows in the cavity and on the lowersurface of the substrate. In reality, polysilicon can also grow on theupper surface of the perforated plate 6 (depending on the method used)in connection with the growing of the membrane 7. The polysilicon grownon the surface of the perforated plate does not have to be removed, butit can be left where it is, whereby it forms a part of the perforatedplate.

Deviating from the embodiment of FIGS. 4 a to 4 f, the insulating layercan be made rougher prior to the growing of the membrane by etching theinsulating layer through the cavity. Thus, the surface of the membranebecomes rough after growing. This brings about the advantage that themembrane does not adhere to the perforated plate during the use of thepressure sensor as easily as a smooth membrane. The tension of themembrane 7 can be controlled with thermal treatment, and the alloy canbe made conductive during the growing stage or afterwards with ionimplantation or diffusion. The membrane 7 forms an electrical contactwith the substrate 4.

If an air gap thicker than the insulating layer 5 of the SOI wafer is tobe achieved between the membrane 7 and the perforated plate 6 in thedifferential pressure sensor, the thickness of the insulating layer canin such a case be increased through the cavity 10 prior to the growingof the membrane 7. The thickness of the insulating layer can be grownwith CVD oxide, for example.

FIG. 4 d illustrates formation of perforations in the perforated plate6. In FIG. 4 d, the preform is seen as a side-section (left-handdrawing) and from above (right-hand drawing).

Perforations are formed in the perforated plate 6 by etching. At first,the desired pattern is patterned litographically on the surface of theplate, using two-sided focusing. After this, the perforations are etchedon the plate 6 as far as to the insulating layer 5. Anisotropicwet-etching or ICP etching can be used in the etching. The ICP etchingprovides the advantage that the perforation can be made optimally dense,and in addition, the etching can utilize what is called a notchingphenomenon of the ICP etching, whereby the acoustic resonance of the airgap of the microphone is reduced (cf. FIG. 6). Further, the adherencerisk of the flexible membrane is reduced owing to the smaller contactarea.

Deviating from the right-hand drawing of FIG. 4 d, the perforated backplate 6 can also be made bridge-like, as shown in FIG. 3.

FIG. 4 e illustrates removal of the insulating layer. The insulatinglayer is removed by etching in a solution which etches out theinsulating layer from the area between the membrane and the perforatedplate, but which does not essentially remove material from the substrateof the perforated plate. The material of the insulating layer in the SOIwafer can be for instance silicon dioxide, in which case for instancethe HF solution (hydrofluoric acid) or the PSG solution (ammoniumfluoride, acetic acid, water) are suitable for its removal.

The etching relating to the removal of the insulating layer alsoproceeds below the outer edges of the perforated plate in the sidedirection as much as in the area of the flexible membrane, but becausethe distance between the perforations of the perforated plate is only afew micrometers, this phenomenon is not detrimental. After the removalof the insulating layer, the membrane 7 is disengaged in such a way thatit can move (stretch) in the cavity.

FIG. 4 f illustrates metallization of a differential pressure sensormanufactured with a method according to the invention. Since arelatively thick layer of an SOI wafer is used as the perforated plate6, the metallization can be performed as the last step without a mask.The metallization can be done by sputtering a thin layer of metal (e.g.aluminium) without the stress or thermal expansion of the metal beingdetrimental. An appropriate thickness for the metal is half of thethickness of the insulating layer, whereby the metallization is notcapable of short-circuiting the perforated plate 6 and the substrate 4.At the points of the perforations in the perforated plate, metal spotsare formed on the membrane 7 as well, but not forming a continuosmembrane, the metal spots do not cause significant stress.

Deviating from the above, the metallization can be performed with amask, whereby no metal spots are formed on the membrane, and thethickness of the metal layer can be increased without the risk that itwould short-circuit the plate 6 and the substrate 4. At least inconnection with a bridge-type perforated plate, there is a reason to usea mechanical mask, in which case contact metal is only gathered in thearea of the contact pads 8.

FIGS. 5 a to 5 d illustrate a second preferred embodiment of a methodaccording to the invention. The other parts of the embodiment of FIGS. 5a to 5 d correspond to the embodiment of FIGS. 4 a to 4 f, but insteadof the kind of cavity formation implemented in FIG. 4 b, the cavity isformed through the intermediate steps of FIGS. 5 a to 5 c.

In FIG. 5 a, a plurality of rings 12 are etched within each other in thesubstrate in such a way that they extend through the substrate 4 as faras to the insulating layer 5. The rings can be etched for example byusing ICP etching.

In FIG. 5 b, the thickness of the insulating layer 5 has been madethinner by etching, whereby some material has been removed through therings 12. After this, the step according to FIG. 5 c follows, in whichthe walls between the rings 12 are removed with wet-etching in such away that a continuous cavity 10 is formed in the substrate.

In FIG. 5 c it is seen that grooves have been formed on the surface 5 ofthe insulating layer. When at the following manufacturing stage themembrane 7 is grown on the surface of the insulating layer 5 (in the waycorresponding to that described in connection with FIG. 4 c), acorrugated, i.e. a winding membrane 7, is provided. When the membrane issubsequently disengaged from the insulating layer above it by etchingout the insulating layer in the way described in connection with FIG. 4e, the result is that with the embodiment according to FIGS. 5 a to 5 d,a membrane is provided which has less stress than the membrane of thedifferential pressure sensor manufactured in accordance with FIGS. 4 ato 4 f.

FIG. 6 illustrates decreasing of the acoustic resistance of a microphoneaccording to the invention. FIG. 6 shows the lower part of theperforated plate 6 and the membrane 7. In FIG. 6, arrows indicate theflow of air in the slot between the plate 6 and the membrane 7. In theleft-hand drawing, the angles of the walls between the perforations ofthe perforated plate 6 are acute, whereby the air flow is poorer than inthe right-hand drawing, in which the angles 14 are blunt.

The blunting of the angles 14 is based on the notching phenomenonmentioned in connection with FIG. 4 d. The notching phenomenon meansthat over-etching results in side-directed cavities. Such cavities areprovided when the ICP etching of FIG. 4 d, in which perforations areformed in the perforated plate, is carried on for a sufficiently longtime, in other words even after the perforations have reached theinsulating layer.

FIGS. 7 a to 7 i illustrate a third preferred embodiment of a method anddifferential pressure sensor according to the invention. The embodimentof FIGS. 7 a to 7 i differs from the one of FIGS. 4 a to 4 f in such away that in the case of FIGS. 7 a to 7 i, additional steps areperformed, which are illustrated in connection with FIGS. 7 b to 7 c andowing to which an additional electrode is provided in the differentialpressure sensor.

FIG. 7 a shows a layer-structured SOI wafer which can be utilized in themanufacture of a differential pressure sensor. Thus, it is a preformcorresponding to the one in the embodiment of FIGS. 4 a to 4 f.

In the step of FIG. 7 b, the manufacture of the differential pressuresensor is started by perforating the perforated plate 6″ of thedifferential pressure sensor by etching, in the manner corresponding tothat described in connection with FIG. 4 d. In the left-hand drawing ofFIG. 7 b the SOI wafer is seen as a side-section and in the right-handdrawing as a top view. In the case of FIG. 7 b, such a pattern is formedon the perforated plate 6″ that comprises an armature area 15 situatedin the middle part of the perforated plate 6″ and separated from therest of the layer.

In the step of FIG. 7 c, the perforations of the perforated plate 6″ andthe groove surrounding the armature area 15 are filled by growing withCVD oxide, for example. The oxide forms at the same time a secondinsulating layer 16 upon the perforated plate 6″.

After the growing of the second insulating layer 16, the manufacture ofthe differential pressure sensor is continued with steps correspondingto those described in FIGS. 4 b to 4 f. At first, the cavity 10 isetched in the step of FIG. 7 d. In the left-hand drawing of FIG. 7 d thedifferential pressure sensor is seen as a side-section and in theright-hand drawing from below.

In connection with the manufacture of a microphone (which is a specialembodiment of a differential pressure sensor), the structure must beprovided with a pressure-balancing opening that opens to the cavity 10.The pressure-balancing opening 9 can be etched in connection with theetching of the cavity 10 from the cavity 10 through the substrate 4 onthe outer surface of the substrate, as shown in FIG. 7 d. Alternatively,if it is not desirable to arrange the pressure-balancing opening throughthe substrate, as in the figure, it can be implemented in accordancewith the invention in such a way that the membrane 7 is provided with aperforation that allows air to flow between the spaces between the upperand lower parts of the membrane. Such a pressure-balancing perforationcan be made in the membrane for instance by puncturing with laser, oralternatively, by etching from the upper or lower part of the membrane,using a photoresist mask.

In the step of FIG. 7 e, a flexible membrane 7 is grown of polysiliconin the way corresponding to that described in connection with FIG. 4 c.It is seen from FIG. 7 e that in connection with the growing of theflexible membrane 7, a second membrane 17 grows at the same time ofpolysilicon upon the second insulating layer 16.

In the step of FIG. 7 f, the second flexible membrane 17 is grown uponthe second insulating layer 16 in such a way that a perforatedadditional electrode is formed of the flexible membrane 17. Theleft-hand drawing of FIG. 7 f shows the differential pressure sensor asa side-section and the right-hand drawing as a top-view.

In the step of FIG. 7 g, the insulating layers 5 and 16 are removed forinstance by etching in an HF solution, in the way corresponding to thatdescribed in connection with FIG. 4 e. Thus, the insulating layers 5 and16 can be removed from the area between the perforations and the cavity10 in such a manner that some insulant 5 and 16 remains in the middle ofthe armature area 15. Thus, the second flexible membrane 17, i.e. theadditional electrode, remains connected to the flexible membrane 7 viathe armature area 15, and the armature area 15, in turn, becomesseparated from the perforated plate 6″.

Finally, in the step of FIG. 7 h, contact metallization is performedwith a mechanical mask, whereby contact pads 8 can be formed in thedifferential pressure sensor. FIG. 7 f shows the differential pressuresensor in the left-hand drawing as a cross-section and in the right-handdrawing as a top view.

The changing capacitance C_(meas) of the additional electrode structureformed by the second flexible membrane 17 is measured from the contactpads 8 between the perforated plate 6″ and the second flexible membrane7 in accordance with FIG. 7 i. Since the second flexible membrane 17 isconnected to the middle of the flexible membrane 7 via the armature area15, it moves as a plane-like panel as much as the most flexible middlepart of the flexible membrane 7. Thus, greater capacitance modulationand improved separating capacity and sensitivity are achieved.

In order to form an electrical contact, the additional electrode formedby the second flexible membrane 17 is patterned in such a way that athin conductor 18 extends from the perforated area thereof to thecontact pad, as seen in FIG. 7 h, for instance. The conductor 18 must beslender so as to prevent the movement of the second flexible membrane 17as little as possible. In order to prevent torsion, it is desirable thatthere are conductors symmetrically placed. The shape of the secondflexible membrane 17 illustrated in FIG. 7 h and the arrangement of itsfour conductors are exemplary; arms having other shapes can be used aswell.

FIG. 8 illustrates a fourth preferred embodiment of a differentialpressure sensor according to the invention. The embodiment of FIG. 8deviates from the preceding embodiments in that it utilizes a substrate4′, which does not conduct electricity. Thus, the flexible membrane 7 isconnected to an electricity-conducting base 8′, which forms a firstcontact pad of the membrane sensor according to FIG. 8. A second contactpad 8 is, by contrast, arranged on the surface of the perforated plate,as described in connection with previous embodiments.

Although it has been explained in connection with previous embodimentsof the invention that a conductive substrate is used in the differentialpressure sensor, it is to be noted that this is only one option. Insteadof a conductive substrate, a conductive base like the one in FIG. 8 canbe used in the embodiments described above, the base being connected tothe flexible membrane, and thus also a substrate that does not conductelectricity can be used.

FIGS. 9 a to 9 f illustrate a fourth preferred embodiment of a methodand differential pressure sensor according to the invention. Theembodiment of FIGS. 9 a to 9 f corresponds to a great extent to theembodiment of FIGS. 4 a to 4 f. The most significant difference is thatin the case of FIGS. 9 a to 9 f, a part of the steps relating to themethod are performed in a deviating order, in addition to which, a fewextra steps are performed to form adhesion prevention collars.

In the embodiment of FIGS. 9 a to 9 f, the differential pressure sensorcan be made of an SOI wafer corresponding to that described inconnection with FIGS. 4 a to 4 f. In the step of FIG. 9 a, perforationsare made in the perforated plate 6 by etching.

In the step of FIG. 9 b, the insulating layer 5 formed of silicondioxide, for example, is etched shortly, i.e. in such a way that pitsare formed in it at the points corresponding to the points havingperforations in the perforated plate 6.

In the step of FIG. 9 c, a cavity 10 is etched in the substrate 4, asdescribed in connection with the previous embodiments.

In the step of FIG. 9 d, a flexible membrane 7 is grown of polysilicon,for instance, for the insulating layer through the cavity 10.Simultaneously, a layer 19 of polysilicon grows on the perforated plate6 and on the visible upper surface of the insulating layer 5 seen inFIG. 9 d. On the upper surface of the insulating layer, a layer 19 ofpolysilicon only grows at the points of the perforations of theperforated plate 6.

In the step of FIG. 9 e, a polysilicon layer 19 is etched out from theupper surface of the perforated plate 6 seen in FIG. 9 e. The etchingcan be implemented as plasma etching. Plasma etching typically etcheshorizontal surfaces much more rapidly than vertical surfaces. Thus, somepolysilicon remains on the side surfaces of the perforated plate 6.

In the step of FIG. 9 f, the insulating layer 5 has been removed, forexample by etching. It is seen from FIG. 9 f that the polysilicon layer19 remaining on the side surfaces of the perforations of the perforatedplate 6 protrudes to some extent from the perforated plate 6 towards themembrane 7. These protrusions of the polysilicon layer 19 thus formcollars that reduce the adhesion sensitivity of the membrane 7, in otherwords they prevent the membrane from adhering to the perforated plate 6.

It is to be understood that the above description and the relatedfigures are only intended to illustrate the present invention. Differentvariations and modifications of the invention will be obvious to aperson skilled in the art, without deviating from the scope and spiritof the invention defined in the attached claims.

1. A method of manufacturing a membrane sensor, comprising selectinginto use a layer-structured preform comprising a first layer, a secondlayer made of conductive material, and an insulating layer between thefirst and second layers; removing material from the first layer byetching a cavity in the first layer, the cavity extending through thefirst layer as far as to the insulating layer; growing a membrane ofconductive material on the insulating layer through the cavity formed inthe first layer, the membrane joining to the walls of the cavity,forming a dense wall in the cavity; removing material from the secondconductive layer by etching perforations in the second conductive layer,the perforations extending through the second conductive layer as far asto the insulating layer and being situated in a finished membrane sensorat the point of the cavity but on the side opposite to the insulatinglayer; and removing the insulating layer from the area between theperforations and the cavity.
 2. The method of claim 1, wherein the firstlayer being of conductive material and by further metallizing in themethod an area in the first and second layers to form contact padsenabling electrical connections.
 3. The method of claim 1, furthercomprising selecting in the method a layer-structured SOI preform intouse, the first layer of which preform is formed of a silicon substrateupon which an insulating layer has been arranged and the secondconductive layer of which is formed of a silicon layer.
 4. The method ofclaim 1, further comprising growing said membrane of polycrystallinesilicon.
 5. The method of claim 1, further comprising enclosing themembrane sensor in the enclosure of an integrated circuit.
 6. The methodof claim 1, further comprising making the insulating layer rougher inthe method by etching through the cavity prior to the growing of themembrane.
 7. The method of claim 1, wherein: the etching of theperforations in the second layer is performed prior to the removal ofmaterial from the first layer, whereby an armature area separated withgrooves from the other parts of the second layer is formed in the middlepart of the second layer; growing a second insulating layer upon thesecond layer when the second layer has been perforated; and after theformation of the cavity, a second conductive membrane is grown upon thesecond insulating layer in connection with the growing of the conductivemembrane, and openings are etched in said second membrane which extendthrough the second membrane to the second insulating layer, whereby inconnection with the removal of the insulating layer, also the secondinsulating layer is removed in such a way that a differential pressuresensor is provided having two flexible membranes connected to each othervia the armature area.
 8. A differential pressure sensor comprising aflexible membrane made of conductive material, which membrane forms afirst electrode of the differential pressure sensor; and a perforatedplate made of conductive material, which plate is essentially more rigidthan said membrane, which plate is arranged at a distance from themembrane, and which plate forms a second electrode of the differentialpressure sensor, wherein the differential pressure sensor also comprisesa substrate, a cavity extending through the substrate, the walls ofwhich cavity are formed of said substrate; the membrane is closelyjoined to the walls of the cavity, whereby the membrane forms a densewall in said cavity, and said perforated plate is attached to thesubstrate with an insulating layer.
 9. The differential pressure sensorof claim 8, wherein the differential pressure sensor is made of alayer-structured SOI preform, whereby the substrate is formed of asilicon substrate of the SOI preform; the insulating layer is made of aninsulating layer of the SOI preform; and the perforated plate is made ofa silicon layer of the SOI preform.
 10. The differential pressure sensorof claim 8, wherein said membrane is made of polycrystalline silicon.11. The differential pressure sensor of claim 8, wherein the area of theperforated plate is essentially smaller than the area of thecross-section of the cavity; and when the differential pressure sensoris seen from the direction of the perforated plate, the perforated areaof the perforated plate is situated above the middle part of the movingarea of the flexible membrane, and from said perforated area, only armssupporting said perforated area extend past imaginary extensions of theside walls of the cavity.
 12. The differential pressure sensor of claim8, wherein a second flexible membrane, which is perforated, has beenattached to the perforated plate on the side opposite relative to thesubstrate with a second insulating layer, and an armature area separatedfrom the other parts of the perforated plate with a groove has beenformed in the middle part of the perforated plate, which armature areais connected to the flexible membrane and to the second flexiblemembrane in such a way that the movement of the flexible membrane istransmitted through the armature area to the second flexible membrane,which moves with the membrane.
 13. A microphone comprising: a flexiblemembrane made of conductive material, which membrane forms a firstelectrode of the microphone; and a perforated plate made of conductivematerial, which plate is essentially more rigid than said membrane,which plate is arranged at a distance from the membrane, and which plateforms a second electrode of the microphone, wherein the microphonefurther comprises a substrate, a cavity extending through the substrate,the walls of which cavity are formed of said substrate; the membrane isclosely joined to the walls of the cavity, whereby the membrane forms awall in said cavity; and said perforated plate has been attached to thesubstrate with an insulating layer.
 14. The microphone of claim 13,wherein the microphone is made of a layer-structured SOI preform,whereby the substrate is formed of a silicon substrate of the SOIpreform; the insulating layer is made of an insulating layer of the SOIpreform, and the perforated plate is made of the silicon layer of theSOI preform.
 15. The microphone of claim 13, wherein said membrane ismade of polycrystalline silicon.
 16. The microphone of claim 13, whereinthe area of the perforated plate is essentially smaller than the area ofthe cross-section of the cavity; and when the microphone is seen fromthe direction of the perforated plate, the perforated area of theperforated plate is situated above the middle part of the moving area ofthe flexible membrane, and from said perforated area, only armssupporting said perforated area extend past the imaginary extensions ofthe side walls of the cavity.
 17. The microphone of claim 13, wherein apressure-balancing opening opens to the cavity, which pressure-balancingopening extends from the cavity through the substrate to the outer wallof the substrate, or which pressure-balancing opening is formed of aperforation in the membrane, the perforation connecting the space abovethe membrane to the space below the membrane.
 18. The microphone ofclaim 13, wherein a second flexible membrane, which is perforated, hasbeen attached to the perforated plate on the side opposite relative tothe substrate with a second insulating layer, and an armature areaseparated from the other parts of the perforated plate with a groove hasbeen formed in the middle part of the perforated plate, which area isconnected to the flexible membrane and to the second flexible membranein such a way that the movement of the flexible membrane is transmittedthrough the armature area to the second flexible membrane, which moveswith the membrane.